3 Layout Planning and Design
4.6 Design Rules for Power Electronic Circuits
If the circuit is characterized by low power requirements and signals with slow rise times and large signal levels, the power distribution system may not be too critical. But as many of these factors change, power conditioning requirements increase, and effective methods for distributing power around a PCB and handling large heat dissipation devices and conductors need careful design effort.
The design of power electronic circuits is more critical than common electronic circuits. This is because comparatively high power is flowing on these PCBs and a failure occurring under the operational conditions on such boards can easily lead to far more serious consequences, including danger to personnel. Several factors need to be considered in the designing of power electronic circuit PCBs. These are discussed below.
4.6.1 Separating Power Circuits in High and Low Power Parts
In power electronic circuits, the circuit which carries less than 3 amp current can be considered as a low power circuit. The circuits which carry more than 3 amps are to be considered as high power circuits. Generally, a control circuit of a considerably low power level controls an active high power electronic component. For example, it is common that a TTL circuit drawing less than 1 amp at a voltage of only 5V may be controlling a thyristor through which the current flow may be as high as 50 amp. One may normally plan to have both the power conditioning and its control circuit on one PCB. However, Figure 4.36 illustrates, in a simplified way, an SCR control circuit. It may be noted that here even the pulse transformer, which provides isolation, is mounted on the high power part of the PCB and not a control PCB, as its secondary winding is driving the high power SCR circuit. If we design both low and high power circuits on one PCB, it will result in capacitive and inductive coupling between power circuits and control circuits, leading to malfunctioning of the equipment. Therefore, the low and high power circuits should be designed on separate PCBs.
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Control PCB
+Vcc
–Vcc
Pulse Transformer
SCR Power PCB
High-power Path
Load
Fig. 4.36 Division of circuit into high- and low-power parts (after Bosshart, 1983)
4.6.2 Base Material Thickness
Power electronic devices dissipate a certain amount of heat which usually needs suitable heat sinks.
If the heat sink is directly mounted on the PCB, the whole board will be heated up to the same temperature. Therefore, the base material selected must withstand the continuous operation of the equipment. A very common choice is glass epoxy laminates. The most used laminate has a thickness of 1.6 mm; however, 2.4 mm and 3.2 mm will thicknesses meet the mechanical property requirements for mounting of heavier components such as pulse transformers, heat sinks, chokes, etc. Heat sinks are now available as pastes for printing.
4.6.3 Copper Foil Thickness
A copper clad laminate with a 35 μm standard thickness is preferable for low power circuits. For high power circuits, normally a copper clad laminate with a 70 μm thick copper foil is commonly used. For special cases, even 105 μm thickness of copper foil may be required.
4.6.4 Conductor Width
In the design of power electronic PCBs, the copper available on the board surface should be fully utilized for the larger currents. The procedure is to first determine the required spacing between the conductors and then allot the remaining copper area to the conductors. Conductors carrying large current should be designed with large conductor width. It is also necessary to analyse the circuit to determine the most probable circuit failures and the conductors likely to be affected on the PCB. A
check must be carried out to ascertain that they can carry the fault current. If, not, the conductor width may be increased as far as possible.
4.6.5 Resistive Drop of Voltage
In power electronic circuits, high currents flowing through the PCB conductors can cause a considerable voltage drop. Wherever possible, these heavy load currents should be avoided on the PCB. In case it is unavoidable to bypass the load current and it has to be carried through the PCB, the conductor should be so designed that the voltage drop caused thereby should not have any influence on the functional ability of the circuit.
4.6.6 Thermal Considerations
Heat gets generated on the PCB from two sources: the board itself and the components mounted on it. Since each system (and components) has a maximum temperature of operation, care must be taken to ensure that this temperature is not exceeded. Use of heat sinks, forced air cooling, placement of components as well as the mounting of the board in horizontal or vertical position will affect the temperature of the board and the components mounted on it.
Considering the maximum allowed temperature rise, caused by the copper track, Table 4.1 gives the minimum allowable track width which can be used in order to ensure temperature rise less than 10°C, 20°C and 40°C for various dc currents. It may be remembered that 1mm track width has a safe current rating of a little more than 2 A and this will not cause an excessive voltage drop in the conductors.
Table 4.1 Minimum Cu-track Width (mm) for Temperature Rise of Less than 10°, 20° and 40° C for Various DC Currents
DC Current Amps Temperature Rise °C (35μm copper-foil)
10°C 20°C 40°C
0.5 0.15 mm 0.10 mm 0.06 mm
1.0 0.40 mm 0.25 mm 0.15 mm
2.0 0.80 mm 0.50 mm 0.30 mm
5.0 3.25 mm 1.75 mm 1.00 mm
10.0 8.00 mm 4.70 mm 3.00 mm
These values only determine the increase in board temperature due to static currents. To this must be added any large direct, alternating or switched currents causing significant component heat dissipation. Fortunately, modern EDA tools allow thermal analysis to be undertaken quickly and
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accurately by using the analogy between the current flow and the heat flow. Any analogue electrical simulation program (like SPICE) should be able to model the static and dynamic thermal considerations.